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f you’re one of the many people who sometimes has difficulty staying asleep, you’ll be interested to learn that the root cause may be at least partly prehistoric. A new hypothesis, based on research concerning modern hunter-gatherers, suggests that humans evolved to be somewhat vigilant at night in order to avoid predators and other threats.

The Hadza of Tanzania are modern hunter-gatherers who live according to the natural rhythms of day and night, just as humans did for hundreds of thousands of years. Their sleeping habits mirror those of our early ancestors, with some of them snoozing outside next to a hearth while others crowd into cozy huts made of woven grass and branches.

Once night falls, the scene is not completely silent and tranquil. At any given moment, an elder member of the group may rise to relieve himself outside. Mothers with newborns may need to tend to their infants. In the distance, predators such as leopards and spotted hyenas could prowl around, searching for their next meal.

Certain potential prey animals in Tanzania, like meerkats, take turns doing sentry duty while others sleep, but the Hadza have no such need for designated guards. That’s because someone at any given moment is nearly always awake. The discovery, reported in the journal Proceedings of the Royal Society B, has led to the newly established “poorly sleeping grandparent hypothesis.”

“The idea behind the ‘poorly sleeping grandparent hypothesis’ is that, for much of human history, living and sleeping in mixed-age groups of people with different sleep habits helped our ancestors keep a watchful eye and make it through the night,” lead author Dave Samson, a sleep expert at Duke University, explained to Seeker.

“That is, any time you have a group with mixed ages, some go to bed early, some later,” he continued. “If you’re older, you’re more likely to be a morning lark. If you’re younger, you’re more likely to be a night owl.”

These basic types were clearly in evidence during the study, which involved 33 healthy men and women among the Hadza agreeing to wear a small watch-like device on their wrists for 20 days. The device recorded their nighttime movements from one minute to the next.

Samson and his team observed that Hadza sleep patterns were rarely in sync. Out of over 220 hours of observation, there were only 18 minutes when all adults were sound asleep simultaneously. On average, more than a third of the group was alert, or dozing very lightly, at any given time.

Usually the Hadza participants went to bed shortly after 10 pm and awoke at around 7 am, but some retired as early as 8 pm and woke up by 6 am. Still others stayed up past 11 pm and slept until after 8 am. Daytime naps were frequent.

Conditions while the Hadza slept were family-oriented, to say the least.

“The Hadza do not sleep on raised cots, but simple ground beds that consist of a textile or an animal hide,” Samson said. “Couples and families consistently sleep on the same bed, and the average bed has 2.4 individuals on it, but I measured up to seven on one, including a nursing mom.”

“This proximity, I am sure, leads to a type of shared ‘asynchrony,’ where when one individual wakes up within a hut, it may increase the likelihood of another waking up,” he added.

Several factors are behind the varying sleep habits. As previously indicated, the number of people in a given sleep group matters, as does the timing of snooze phases, such as REM (rapid eye movement) sleep, a deeper rest during which dreaming tends to occur.

The primary driver, however, is each individual’s chronotype.

“Chronotype is defined as the individual propensity for sleep and activity at particular times during a 24-hour period,” Samson explained. “It's also moderately heritable and varies through the lifespan — thus it is controlled by genes, environment, and age.”

As a result, some humans wind up being morning people and others become night owls. Chronotype variation is found not just in primates but also in other animals.

“We believe this research could shift our understanding of age-related sleep disorders because a lot of older people go to doctors complaining that they wake up early and can’t get back to sleep,” Samson said. “The idea here is that maybe there isn't anything wrong with them.”

“Maybe these are not disorders at all but are legacies of our evolutionary past, when they were beneficial,” he continued. “This has important clinical implications, because it could benefit elderly individuals knowing that their sleep pattern is not a disorder but is, in fact, normal.”

During ancient times, being awake at certain times of night could save one from a predator, a natural disaster, or even an attack from another person hoping to catch their victim while he or she is vulnerable.

Nighttime partying every so often may be normal too, at least for some people. Samson slept in the Hadza camp throughout the study. On moonless nights, he could hear a Hadza ritual known as the epeme dance. During these dances, married men and women would divide up into separate groups.

“Generally, the women would sing and dance for a few minutes and the song would crescendo with males clapping and hollering,” he recalled. “Then there would be silence for a time, and the cycle would begin anew. This went on for hours and was mesmerizingly beautiful to listen to from my tent.”

When he and his colleagues looked back on their gathered sleep data, they could immediately detect which nights had an epeme dance and which did not. The researchers could see, via their graphs, all of the activity that went on throughout the dance nights.

Samson said, “It just goes to show how sometimes it’s worth losing sleep to perform socially important rituals that bond the community together.”